A high level of reliability is required of indicator lights that are used in nuclear-reactor control panels as well as in many other types of equipment. The extent to which this requirement is fulfilled is affected not only by the bulb itself but also by its electrical environment. Accordingly, the designer should give serious attention to the design of the switching and supply systems that constitute the electrical environment of the bulb.
Among the types of switching and supply systems currently used is the combination of a transformer with a mechanical relay or switch. The transformer supplies AC power, and the relay or switch controls application of the power to the bulb. This combination is simple, and its use of an AC source makes it more merciful to the bulb filament than it would be if it employed a direct-current source. The mechanical relay or switch, however, is relatively large and unreliable in comparison with the solid-state switch. In contrast to a solid-state switch, moreover, a mechanical switch or relay cannot be activated at zero crossings of the AC supply, and this results in an unfavorable effect on bulb longevity.
The solid-state switch also has disadvantages. Solid-state switches fail when subjected to currents that exceed their rated capacities, and such currents may be drawn by incandescent bulbs when they fail short, as they occasionally do. The shorted bulb draws a large current whose magnitude is limited only by line impedances. Therefore, if a solid-state switch has been chosen that has a continuous-current rating not greatly in excess of the normal bulb current, its short-duration current rating may well be exceeded by the current drawn by the shorted bulb. The short-duration current rating of the solid-state switch can also be exceeded when a cold bulb is turned on. Accordingly, in order to avoid damage to the solid-state switch, the designer must choose switches with continuous-current ratings considerably in excess of the normal bulb current. This, of course, increases expense and reduces the advantages that solid-state switches would otherwise have over mechanical switches and relays.
In order to avoid choosing a solid-state switch with a continuous-current rating greatly in excess of the normal current requirement of the bulb that it controls, designers have protected the solid-state switch by providing a fuse in series with it. Fuses provide only limited protection, however, since they cannot always be relied on to blow before the switch has been damaged. In addition, the use of a fuse makes it necessary to replace both the fuse and the bulb if the bulb fails short. Furthermore, when several indicator lights are controlled by the same switch, the use of fuse makes it necessary to change the fuse before the defective bulb can be located.
A supply and switching system that protects solid-state switches without using fuses is the combination of a solid-state switch with a current-limited DC power supply. Though simple in concept, this approach greatly increases the cost in systems in which large numbers of bulbs are to be powered. In addition, this type of system exhibits the unfavorable bulb-longevity characteristics of a DC supply.
It is apparent that the present state of this widely used and well explored art requires the designer to make trade-offs among bulb longevity, circuit reliability, and expense because the art has provided no means for obtaining the optimum in all three.